The present invention relates to rollers, more specifically, printing rollers. However, the cover and method of the present invention may be applied to any similar structures comprising a plastic coated core, whether or not these are used as printing rollers.
Traditionally, printing rollers have been constructed from a metal core surrounded by a soft rubber coating. Even the highest quality rubber roller begins to deteriorate from the first date of use. Because the rollers are rubber, they are vulnerable to attack by inks and solvents normally used in printing. Maintenance is costly, both in labor required to maintain them and, more importantly, in the down time of the presses.
A major problem with rubber rollers is end swell. This occurs when solvents acting on both the ends of the roller and the surface swell the ends more than the center. Uneven ink carrying ability, improper split of the ink film, poor quality printing and lack of sharpness are often the results of end swell. End swell can be corrected by regrinding the roller, but this solution is only temporary, as the same factors which caused end swell originally are probably still in existence. Other problems of rubber rollers are outlined in L. Harrison, "Roller Maintenance Pays Off", Reproductions Review & Methods, October, 1975, pp. 48-49.
There have been attempts in industry to attach a 0.020 inch thick polytetrafluoroethylene or FEP sleeve to a roller by heat shrinking the sleeve around the core. These attempts have not been entirely successful. As the roller is used, the plastic sleeve tends to separate from the roller or move toward one end or the other. With a sleeve of this thickness, a ding, a small dent, tends to remain in the plastic cover. The impression of the ding is sometimes transferred to the printed web.
Aser et al., in U.S. Pat. No. 3,435,500, disclose a steel core roller covered with a cylinder of silicone rubber. This cylinder is then covered with a sleeve of Teflon FEP, duPont's fluorinated ethylene/propylene. The Teflon FEP sleeve is then etched and primed on its inner surface and heat shrunk around a rubber mandrel after the mandrel has been coated with a suitable silicone rubber adhesive. The roller is intended for use primarily in the field of xerography as the heated pressure fusing device used in fusing resinous powder images onto sheets of paper to which which the powder images have been transferred after being formed by deposition of powder on an electrostatic latent image. While the Teflon FEP coated roller is an improvement over the prior art rubber rollers, there are some disadvantages associated with this as well. Also, the adhesives used in the Aser patent are pressure sensitive adhesives which remain tacky during the assembly stage making assembly difficult and inviting dirt and foreign particles to be entrapped between the sleeve and roller. Such tacky adhesive prevents the shipping of the mandrel separately from the Teflon FEP sleeve after the adhesive has been applied to both. It is a decided advantage to be able to ship both parts separately, so that the sleeve alone may be applied to an existing roller for repair purposes. More importantly, since the adhesive never hardens, the cover tends to creep, especially when the roller is operating at high speeds.
Shoffner, in U.S. Pat. No. 3,749,621, bonds a heat shrinkable Teflon FEP sleeve to a roller by a melt bonding process. By this process, the inside surface of the Teflon FEP sleeve is heated to its melting point and pressure bonded to the roller surface. Although Shoffner discloses that a suitable adhesive, dielectrically activated, may be used, it is the melt bonding of the sleeve itself by which the Teflon FEP sleeve is bonded to the roller. The disadvantages of this method are that the temperature must be very carefully controlled so as not to cause degradation of the Teflon FEP. Control devices are very expensive and the process is not adapted for covering rubber coated rollers, as the high temperature required to melt the Teflon FEP would cause the rubber to deteriorate.
Holmes et al., in U.S. Pat. No. 3,481,805, disclose the bonding of a polytetrafluoroethylene (hereinafter PTFE) sleeve over a silicone rubber coated metal roller. The PTFE sleeve, after preliminary etching, is heat shrunk around the roller which was coated with a silicone resin adhesive. As pointed out at column 3, lines 3-4, this adhesive is wet and tacky and the roller should not be touched except at its uncoated metal ends. Thus, it is difficult to orient the sleeve in relation to the roller in Holmes et al.
Heslop et al., in U.S. Pat. No. 3,539,411, disclose a heat recoverable jacket having a mastic coated on its inner surface and further having an anti-tack coating on the inner surface of the mastic, which is used to cover pipe joints. The jacket structure is then slipped over the end of a pipe until it reaches the joint and then heat shrunk to seal the joint. The anti-tack coating goes into solution with the mastic under the temperature conditions used in heat shrinking and it stays in the solution at service temperatures. Although the heat recoverable jacket may be PTFE, the addition of an anti-tack coating to the inner surface of the mastic layer is more expensive than the tack-free adhesive used in the present invention. The greater expense results from the requirement of an additional coating as well as the time and machinery required to apply the extra anti-tack coating. Another disadvantage is that if the proper temperature is not maintained so that the anti-tack coating stays in solution with the mastic, the jacket may separate from the pipe. Such separation would be more likely where the article covered was not a pipe but rather a moving printing roller.
Other patents which relate to the coating of rollers of which applicants are aware are U.S. Pat. Nos. 3,470,046, 3,677,845, 3,826,702 and U.S. Pat. No. Re. 27,028.